1. Field of the Invention
The present invention relates to a network relay device that relays data in a network.
2. Description of the Related Art
The network generally adopts the redundant structure, in order to prevent the shutdown of the entire network due to the occurrence of some trouble or failure in a line or a device included in the network.
A layer 2 (data link layer) and a layer 3 (network layer) in the OSI reference model have different network characteristics. In the case of an internet protocol (IP) as a typical example of the layer 3, simple connection of two devices by a physical line does not establish intercommunication between the connected devices unless IP addresses are set in the respective devices. In the case of Ethernet (registered trademark) as a typical example of the layer 2, on the other hand, simple connection of two devices by a physical line establishes intercommunication between the connected devices.
An L2 switch is one example of layer 2 (L2)-compliant network relay devices. Interconnection of multiple L2 switches by two or a greater number of physical lines or establishment of the redundant structure by multiple L2 switches undesirably causes a loop in the network as shown in FIG. 7. In the illustration of FIG. 7, SW represents an L2 switch.
The occurrence of the loop in the network may lead to a significant problem. In the process of transmission of packets via the Ethernet (registered trademark), for example, in the case of an unknown destination (MAC address) of the packets, the L2 switch uses a broadcast address and sends broadcast packets. The L2 switch sends the broadcast packets to a line other than the packet-receiving line and does not memorize the packets transmitted previously. In the presence of a loop in the network, the broadcast packets infinitely multiply and endlessly circulate in the loop. This applies significant loading to the packet transfer of the L2 switch and affects other networks to occupy the available bandwidth.
A known measure against this potential problem divides the multiple lines and multiple L2 switches constituting the loop into an active system and a standby system. This technique blocks off the packet transfer from the L2 switches in the standby system and disconnects the lines in the standby system, while establishing communication only by the L2 switches and the lines in the active system. This prevents the occurrence of a loop in the network.
In the case of virtual LANs (VLANs), setting of the active system or the standby system is carried out with regard to each group of a single or multiple virtual LANs. Namely the L2 switches and the lines working as the active system in one group may be specified as the standby system in another group, and vice versa.
A prior art system may use a pair of the L2 switches, which are interlocked to effect a switchover between a master mode and a backup mode as shown in FIG. 8. This structure enables a smooth switchover from the active system to the standby system, in the event of any trouble or failure arising in the current active system.
In the system using the paired L2 switches, one of the L2 switches is set to the master L2 switch and is used in the active system, whereas the other of the L2 switches is set to the backup L2 switch and is used in the standby system. The master L2 switch opens a port to establish communication via a line linked to the port, while the backup L2 switch blocks off the port to disconnect the line linked to the port. During operations, the master L2 switch and the backup L2 switch send and receive the control frame signals to and from each other at regular intervals for mutual confirmation of the effective presence. In the event of any trouble or failure arising in the master L2 switch or in the line connecting with the master L2 switch, the backup L2 switch detects the occurrence of the trouble or failure and takes over the master mode in place of the current master L2 switch to open the blocked port and establish communication via the line linked to the port. This leads to a switchover of the entire network from the active system to the standby system.
The network-related technique described above is disclosed, for example, in U.S. Pat. No. 5,473,599.
As mentioned above, the master L2 switch and the backup L2 switch send and receive the control frame signals to and from each other at regular intervals for mutual confirmation of the effective presence. A control module in each of the L2 switches manages the control frame signals. The control module in the master L2 switch may fall into some busy status. The communication module generally carries out the hardware processing and thus attains normal transfer of packets even in the busy status. The control module in the busy status may, however, fail to send the control frame signal to the backup L2 switch. In response to failed reception of the control frame signal from the master L2 switch, the control module in the backup L2 switch wrongly detects the occurrence of a failure or trouble in the master L2 switch and takes over the master mode in place of the current master L2 switch to open the blocked port and establish communication via the line linked to the port. This causes both the paired L2 switches to work in the master mode and fall into a double master situation. Communication is established individually via the paired L2 switches in the master mode. This undesirably causes a loop in the network.